Delete tests/codegen/fmt_int_no_panic.rs

this test was included for demonstrative and discussion purposes but does not test what its name alleges - in fact it does not test much of value at all!

as mentioned in this comment, #122770 (comment) , writing a correct test for this codegen outcome is difficult (partially because the public interface to std::fmt intentionally includes an #[inline(never)] function!), so to test this correctly will require more than i can offer in 122770.